reproduction in certain media and at certain temperatures—are scattered everywhere in the atmosphere. Interesting inquiries into their distribution in air and water have been made by Dr. Miquel at the Montsouris Observatory, Paris, and by Dr. Percy Frankland in England. Dr. Frankland has found that the number present is much reduced in winter. Experiments made in inclosed places, where there is little or no aërial motion, show the number of suspended organisms to be very moderate; but as soon as any disturbance in the air occurs, from draughts or people moving about, the number rapidly increases and may become very great. Being slightly heavier than air, they have an invariable tendency to fall, and on that account collect on the surface of water. Hence rivers, lakes, and ponds are constantly being thus contaminated.
Important points connected with dust of organic origin are its inflammability and its liability to explode when mixed with air. The property of explosiveness was forcibly illustrated in the destruction of six flour-mills by this cause in Minneapolis, Minn., in May, 1878. Coal-dust in coal-mines is a cause of accident from explosions which has been closely investigated in England, Germany, and other mining countries. The subject was thoroughly treated by Sir Frederick Abeel, in a paper on Accidents in Mines, read before the Institution of Civil Engineers in 1888.
Extremely fine particles of mineral dust may exist in the atmosphere, and do exist there more frequently than is generally thought, while they escape detection by our senses. The author, while making experiments on the Peak of Teneriffe, in 1878, found the knife-edges of his balance so clogged with this invisible dust that the balance refused to act. When wiped off, the dust collected again in a few minutes, and it was only by continually wiping it away that he was able to go on with his investigation. Prof. Piazzi Smyth, while on the Peak of Teneriffe, witnessed strata of dust rising to a height of nearly a mile, reaching out to the horizon in every direction, and so dense as to hide frequently the neighboring hills. Prof. S. P. Langley, looking down from the height of fifteen thousand feet on Mount Whitney, California, into a region that had appeared clear from the valley below, saw "a kind of level dust ocean, invisible from below, but whose depth was six or seven thousand feet, as the upper portion only of the opposite mountain-range rose clearly out of it."
Dust storms are classified by Dr. Henry Cook, according to their intensity, as atmospheric dust, dust columns, and dust storms. Dr. Cook has observed in India that there are some days on which, however hard and violently the wind may blow, no dust accompanies it, while on others every little puff of air or current of wind forms or carries with it clouds of dust. If the wind which
